The invention relates to a knife assembly for a veneer lathe. Conventionally, the knife assembly includes a knife mounting beam and a nose bar beam, both supported by a frame structure. Either one of these elements comprises a beam-like member which extends essentially over the entire length of the lathe and incorporates means required for mounting the opposed blade elements known as the cutting knife and the nose bar. Generally, the opposed sides of the support beam are provided at the fixing points of the blade elements with a plurality of bulkhead-like projecting members that function as stiffeners of the support beam structure. The knife assembly is arranged controllably movable along guides toward the log being peeled in synchronism with the progress of peeling, and, respectively, away therefrom when the peeling of a new log is to be started. The mutual distance between the knife mounting beam and the nose bar beam is made adjustable to control the knife gap between these opposed knife elements. To achieve a good peeling result, it is mandatory to keep the knife gap in a predetermined value over the entire length of the knife mounting beam and the nose bar beam. Hence, accurate control of the knife gap requires both the knife mounting beam and the nose bar beam to be massive structures that are rigid and resistant to bending.
Conventionally, veneer is peeled from soaked wood that has been kept in a water or steam bath in order to elevate the temperature of the log. When entering the lathe station, the temperature of the log may by as high as 80xc2x0 C. Additional heat is generated from the friction of the knife peeling the log and the nose bar running on the log, as well as from the friction of the veneer passing through the knife gap. This heat load is imposed on the knife mounting beam and the nose bar beam within the structures of the beams holding the knife bar and the nose bar. Such a local rise of temperature generates thermal stresses in the knife mounting beam and the nose bar beam that result in minor deformations of these structures. However, the deformations also are reflected in the value of the knife gap that should stay constant to a tolerance of about 0.02 mm over the entire length of the knife and nose bar.
In the prior art a remedy to this problem has been generally sought by way of providing heating means on the rear portions of the knife mounting beam and the nose bar beam that are on the opposite side of the beam relative to the mounting structures of knife and nose bar. Conventionally, such heating has been accomplished by adapting cavities into the reinforcing structures of the rear portions of the beams and then circulating heated medium therein. The goal of these arrangements has been to stabilize the temperature of the entire knife/nose bar assembly at an elevated level. This technique can indeed minimize deformations induced by thermal stresses on the knife mounting beam and the nose bar beam. However, the overall result thus obtained has not been sufficiently well controlled to keep the knife gap at its predetermined value over the entire length of the knife.
A knife assembly, which is implemented according to the invention and comprises in a conventional manner a frame assembly that supports a knife mounting beam and a nose bar beam, both of which having a front portion for holding a cutting knife and a nose bar insert, respectively, and a stiffening rear portion, whereby the knife mounting beam and the nose bar beam are adapted movable relative to each other on the frame assembly so as to permit adjustment of the knife gap formed between them, and further comprises heat transfer means adapted to the stiffening rear portion of both the knife mounting beam and the nose bar beam, offers in accordance with the invention an improvement in controlling the knife gap to a correct predetermined value by virtue of having placed on both the knife mounting beam and the nose bar beam, in a close vicinity to the knife and nose bar at least one first set of temperature sensors disposed in predetermined positions along the length of the knife and nose bar, and, both the knife mounting beam and the nose bar beam having placed thereon at least one second set of temperature sensors, disposed in predetermined positions along the length of the knife and nose bar, at a distance from the knife and the nose bar, and the assembly further having the heat transfer means located in the stiffening rear portion of the knife mounting beam and the nose bar beam, respectively, being adapted controllable for adjusting the respective portions of the knife mounting beam and the nose bar beam under feedback from the second set of temperature sensors to a temperature value derived from the measurement signal given by the first set of temperature sensors.
Next, the invention will be examined in greater detail with the help of the attached drawing, wherein is diagrammatically illustrated a knife assembly of a veneer lathe.